U.S. Pat. No. 4,065,605 discloses tetrahydro-2-nitromethylene-2H-1,3-thiazine ##STR1## and certain N-substituted congeners, having useful insecticidal properties. German patent specification 3,310,744-A and U.S. patent application Ser. No. 578,142 disclose insecticidal N-acyl derivatives of Compound A, having improved physical properties. Hitherto, all of such N-acyl derivatives have been prepared by treating the N-unsubstituted compounds with suitable reagents. However, attempts to prepare certain N-acyl derivatives by direct acylation have failed. Moreover, synthesis of Compound A by the method disclosed in U.S. Pat. No. 4,065,605 involves the use of hazardous intermediates, so that a route to the N-acyl derivatives that does not involve Compound A is desirable.